Encapsulated Phase Research Articles

Preparation and characterization of encapsulated phase change materials in presence of gamma alumina for thermal energy storage applications

Modified gamma alumina/fatty acids composite phase change materials (PCMs) were prepared via encapsulation with poly(St-co-DVB) shell by phase inversion emulsification method. The gamma alumina (γ-Al2O3) nanoparticles were modified by cetyl trimethylammonium bromide (CTAB). The modified gamma alumina (M-γ-Al2O3) was added into fatty acid mixture, which composed of oleic acid (OA) and capric acid (CA) besides to n-hexadecane. Thus, the effect of alumina addition on the morphological and thermal properties of composite PCMs was investigated. The attachment of CTAB onto gamma alumina nanoparticles was verified by thermogravimetric analysis (TGA) method. The morphological and chemical characterization of empty shell material (PS-DVB shell), microencapsulated PCM and composite PCMs was investigated by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy, respectively. Furthermore, the thermal properties of the PCMs were examined by differential scanning calorimetry (DSC), thermogravimetric analyses and heat storage rate measurement.

Encapsulation of paraffin wax by rigid cross-linked poly (styrene divinylbenzene-acrylic acid) and its thermal characterization

The latent heat phase change materials (PCMs) have emerged as an important and effective thermal storage material. The massive change in volume and the requirement for specialized containment of PCMs limits the thermal storage application of the common PCM like paraffin wax. Herein, we report the encapsulation of paraffin wax with the cross-linked poly(styrene-divinylbenzene-acrylic acid) shell via suspension polymerization. The optical and SEM images of the prepared encapsulated phase change materials (EPCMs) were in the size range of 200–500 µm. The thermal charging-discharging rate of the EPCM was studied using a facile water bath technique. It was found that the optimized EPCM showed about 1.7 times faster-charging and 3.5 times faster discharging rate compared to reference PCM (without encapsulation). The latent heat for the EPCM was found to be about 3.3 times higher than the reference PCM. The thermal charging-discharging ability and latent heat values are higher for the encapsulated paraffin wax when compared with reference paraffin wax. The encapsulation of paraffin wax by polymeric materials via facial suspension polymerization using polymerizable vinyl monomers could find its application as heat storage materials.

Use of encapsulated phase change materials in lightweight building walls for annual thermal regulation

The use of phase change materials (PCMs) in building envelopes is a promising energy efficiency technology. Nevertheless, it is difficult to design a PCM wall for annual thermal regulation since the outdoor environment varies significantly in different seasons. To figure out a design strategy for the PCM walls, three wall specimens with different assemblies were designed and developed to test the thermal performance in summer and winter. A mathematical model was established and validated using experimental data. The thermal performance of the proposed walls was tested, simulated and evaluated using internal surface temperature, thermal inertia and daily heat fluxes compared with a reference wall. The internal surface temperature amplitude was reduced by maximum of 0.73 °C by 21.4% during summer and 0.88 °C by 23.9% during winter. The thermal inertial which represents the ability of the wall to resist the change in temperature was enhanced from 1.24 to 2.0 by 60.3%. The annual average energy savings by using PCMs were 17.7%, 20.2% and 23.1% when the PCM spheres were placed at the first, second, and third layer, respectively. The optimal position of PCM spheres was the third layer for summer, fourth layer for winter, and third layer for annual application.

Natural convective flow and heat transfer of Nano-Encapsulated Phase Change Materials (NEPCMs) in a cavity

Free convective flow and heat transfer of a suspension of Nano Encapsulated Phase Change Materials (NEPCMs) in an enclosure is studied. NEPCM particles are core-shell structured with Phase Change Material (PCM) as the core. The enclosure is a square cavity with top and bottom insulated walls and differentially-heated isothermal vertical walls. The NEPCM particles circulate under natural convection inside the cavity. The PCM cores undergo phase change from solid to liquid and absorb some of the surrounding’s heat in the form of latent heat in the hot region, and release the absorbed heat in the cold region by solidification. The governing equations representing the conservation of mass, flow, and heat of NEPCM suspension are introduced in the form of partial differential equations. The governing equations are transformed into non-dimensional form and solved by the finite element method. A grid check and validation test are performed to ensure the accuracy of the results. The outcomes show that the fusion temperature of NEPCM particles is the key factor affecting the heat transfer enhancement of NEPCMs in the natural convection flow. The enhancement of heat transfer is highly dependent on the non-dimensional fusion temperature, θf, and very good performance can be achieved in the range of ¼ < θf < ¾. Comparing to the base fluid, a relative enhancement of about 10% can be achieved by using NEPCMs at a non-dimensional fusion temperature of ¼.

Effect of varying extent of PCM capsule filling on thermal stratification performance of a storage tank

Among the different pathways of improving the energy storage and energy utilization in a thermal energy storage system, the formation of thermal stratification in hot water tanks is a promising technology. In this study, we developed a novel numerical model to assess the thermal stratification performance in a hot water tank due to addition of encapsulated phase change material (PCM) by varying the bed height, bed porosity and the encapsulation diameter. The formulation of the present numerical model is devoid of complicated momentum and energy equations. A set of simplified energy balance equations is developed to account for heat transfer between the heat transfer fluid (HTF) and the PCM considering local thermal non-equilibrium. The temperature profile at the outlet of the storage tank during charging process demonstrates an enhancement in the Richardson number by 58.3%, at the end of the charging process, upon doubling the PCM bed height, and the corresponding improvement in the charging efficiency is found to be 54.6%. The melting point of PCM plays a pivotal role on the extent of stratification and storage efficiency, as it dictates the amount of heat diffusion towards the bottom section of TES. During discharge phase, if the HTF flow rate is doubled from the nominal value of 2 L/min, the extraction efficiency is enhanced by ∼16.7%.

Thermal and economic evaluation of thermocline combined sensible-latent heat thermal energy storage system for medium temperature applications

In the current work a new thermocline combined sensible-latent heat thermal energy storage configuration is proposed as an alternative to the currently used thermal storage systems; containing solid rod structures of cheap naturally occurring material with phase change material capsules impregnated between the rods. An energy balance method coupled with an enthalpy based technique are used to develop a comprehensive transient numerical model. The numerical simulations are performed to compare the performance and the cost of the three types of thermal energy storage systems. The three thermal energy storage systems are; sensible rod structure, encapsulated phase change material and combined sensible-latent heat. The influence of different evaluation indexes on economic feasibility and the performance of thermal energy storage such as capital cost, capacity cost per kWh, axial temperature distribution, pumping work, thermocline degradation, effective discharging time and effective discharging efficiency; are analyzed. The results show that effective discharging efficiency and capacity costs for encapsulated phase change material, combined sensible-latent heat, sensible rod structure are 95%, 87%, 76% and $42/kWh, $37/kWh, $35/kWh, respectively. Moreover, the hybrid configuration exhibits a storage capacity of 78.5 kWh/m3, which is 26% higher than sensible rod structure and 22% lower than encapsulated phase change material configuration. The results of the comparative study indicate that the combined sensible-latent heat TES system seems to be a more viable option among the considered configurations due to its optimized performance and comparatively low cost. Also due to the reasons that thermal ratcheting of the storage tank is avoided and it provides stable fluid outlet temperature.

Encapsulated Phase Change Material Embedded by Graphene Powders for Smart and Flexible Thermal Response

Flexible responsive materials, such as graphene embedded phase change material (PCM), can exhibit a smart response to the heating and cooling steps and be utilized for the thermal sensing applications. The electrical conductivity is varied during the phase transition due to volume change of the working material. However, the leakage is one of the serious problems and restricts the PCM from applications. Phase change material was encapsulated in the present work to prevent the leakage problem. Polyaniline (PANI) was selected as the supporting material to surround the pure phase change material and thus the fabricated PCM composite could sustain the shape under the phase transition process. Herein, the polyaniline encapsulated phase change material was produced and the graphene powder was added to increase the electrical property of the PCM composite. The graphene embedded PCM composite showed excellent electrical performance when the temperature was increased to the isothermal phase transition state. The pure PCM inside the microcapsule began to melt and the liquid state lead to the volume expansion of the PCM composite. Therefore, the reversible form stable phase transition was achieved and the electrical conductivity was increased as the distance between conductive graphene fillers was reduced with volume expansion of the PCM. This study suggests that the microencapsulated PCM composite should provide new applications for thermal sensing and flexible thermo-electric devices like smart photodetectors.

Multiphysics analysis of effects of encapsulated phase change materials (PCMs) in cement mortars

There is growing interest in the use of phase change materials (PCMs) in the building industry, particularly in cementitious materials. However, the phenomenological interactions between PCMs and cement remain poorly understood and need to be clarified.In this study, mechanical tests were performed on several mortars modified by adding different amounts of PCM (5, 10, and 15 wt%).A series of experiments, including thermophysical analyses (thermogravimetric analysis and semi-adiabatic calorimetry), microstructural analyses (X-ray tomography, mercury intrusion porosimetry, scanning electron microscopy) and Fourier transform infrared spectroscopy provided phenomenological explanations of the evolution of the mechanical properties of these composites.It was determined that the degradation of the mechanical performance of the PCM-mortar composites was due to not only weak PCM inclusions but also a decrease in the degree of hydration of the composites, which induced a special evolution of the pore microstructure.

High-intensity ultrasound assisted manufacturing of melamine-urea-formaldehyde/paraffin nanocapsules

The average diameters of encapsulated phase change materials (PCMs) affect their physical properties. In this study, the effects of the main emulsification variables (ultrasound time and output power of ultrasonic treatment) were explored and then melamine-urea-formaldehyde (MUF)/paraffin nanocapsules were obtained via an in-situ polymerization method. The influences of these variables on the stability of the MUF/paraffin nanoemulsions and the appearances, mean droplet diameter and thermos-physical properties of MUF/paraffin nanocapsules were investigated. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analyzer (TGA), and differential scanning calorimetry (DSC) were employed to characterize the morphologies, size distributions, and thermal stabilities of MUF/paraffin nanocapsules, respectively. The characterization results showed that the more stable nanoemulsion could be formed after ultrasonic treatment for 15 min under the output power at 600 W compared with mechanical stirring emulsification. When the ultrasonic output power was 600 W, the MUF/paraffin nanocapsules achieved the highest encapsulation efficiency (35.8%) and encapsulated paraffin in the MUF shell under the output power of 600 W was more than that obtained under other ultrasonic output power values. This study verified that the ultrasound-assisted emulsification process could be used to fabricate stable MUF/paraffin nanocapsules.

Experimental analysis for thermal storage performance of three types of plate encapsulated phase change materials in air heat exchangers for ventilation applications

Due to climate change and rising global temperatures, energy demand associated with commercial and office building cooling, is projected to increase. Passive-cooling, based on phase change materials, can assist to moderate this increase, however relatively few modelling equations to describe its operating behaviour exists. In this experimental investigation the melting performance of three commercially available encapsulated phase change materials were evaluated for passive cooling applications in air ducts systems. Two paraffin type phase change materials and one salt hydrate phase change material with melting temperatures in the range of 22 °C and 28 °C were considered. Vertical orientated plate type encapsulations with a thickness of 10 mm and a pitch of 15 mm were tested for air inlet temperatures ranging from 30 °C to 35 °C and upstream air velocities ranging from 0.4 m/s and 0.9 m/s. The average effectiveness, cooling power, energy absorption, and phase transformation durations were determined. It was found that the average thermal effectiveness decreased with increased velocities and that the cooling power, which was inversely proportional to the phase transformation duration, increased with air flow rate and inlet air temperature. Based on the data a new empirical correlation model was developed which describes the cooling capacity of the in-duct phase change material plates.

Review and selection of EPCM as TES materials for building applications

ABSTRACTIn order to improve the thermal efficiency of building thermal energy storage (TES) systems, the feasibility of using encapsulated phase change materials (EPCMs) as heat storage media is analysed in this work. Specifically, the finite element method is used to perform thermal behaviour analyses of several EPCMs. These analyses include technical and economic assessments in order to identify the best combination of PCM and shell material, using as main parameters: thermal energy storage, heat transfer rate, materials cost, among others. The results show that EPCMs composed by Na2SO4·6H2O as PCM and covered by stainless steel highlight as TES materials.

An investigation on heat transfer performance of polystyrene encapsulated n-octadecane based nanofluid in square channel

The phase change material based nanofluids (PCM nanofluids) is a promising heat transfer fluid for various applications including electronics cooling and thermal power plants. The present work focusses on the study of heat transfer performance of polystyrene encapsulated phase change material (nanoPCM) based nanofluid. The nanoPCM was synthesized by mini-emulsion polymerization in which PCM core (n-octadecane) is encapsulated within polystyrene shell. The nanoPCM was characterized by Scanning Electron Microscope (SEM), Particle Size Analyser (PSA), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared (FTIR) spectroscopy. The PCM nanofluid was prepared by dispersing nanoPCM in deionized water and the thermo-physical properties of the nanofluid were estimated. The optimum concentration of PCM nanofluid for maximum stability was fixed as 0.7% v/v, based on zeta potential measurements. The heat transfer and pressure drop studies of PCM nanofluid were conducted in a square channel of 8 mm hydraulic diameter. The PCM nanofluid shows up to 37% improvement in heat transfer performance as compared to deionized water (DI water). The enhancement in heat transfer coefficient of PCM nanofluid is attributed to the improved heat capacity of nanoPCM and latent heat absorption during its melting. The results reveal that heat flux and flow rate of PCM nanofluid plays a crucial role in heat transfer performance. A slight increase in pressure drop and the pumping power was observed for the PCM nanofluid as compared to deionized water, especially at higher mass flow rates due to the increase in the viscosity of PCM nanofluid.

Hyperencapsulated mucoid pneumococcal isolates from patients with cystic fibrosis have increased biofilm density and persistence in vivo.

Mucoid bacteria, predominately Pseudomonas aeruginosa, are commonly associated with decline in pulmonary function in children with cystic fibrosis (CF), and are thought to persist at least in part due to a greater propensity toward forming biofilms. We isolated a higher frequency of mucoid Streptococcus pneumoniae (Sp) expressing high levels of capsular polysaccharides from sputa from children with CF, compared to those without CF. We compared biofilm formation and maturation by mucoid and non-mucoid isolates of Sp collected from children with and without CF. Non-mucoid Sp serotype 19A and 19F isolates had significantly higher levels of biofilm initiation and adherence to CF epithelial cells than did serotype 3 isolates. However, strains expressing high levels of capsule had significantly greater biofilm maturation, as evidenced by increased density and thickness in static and continuous flow assays via confocal microscopy. Finally, using a serotype 3 Sp strain, we showed that highly encapsulated mucoid phase variants predominate during late adherence and better colonize CFTR-/- as compared to wild-type mice in respiratory infection studies. These findings indicate that overexpression of capsule can enhance the development of mature pneumococcal biofilms in vitro, and may contribute to pneumococcal colonization in CF lung disease.

Numerical investigation on the phase change process of different shaped macro encapsulated PCM

Latent heat energy storage using macro encapsulated phase change material is an emerging technique for thermal energy storage applica- tions. The main aim of the present investigation is to investigate the melting process of phase change material filled in different shaped configurations. The selected different cavities are square, circular and triangular. A mathematical model based on convection dominated melting is required to be developed, especially in view of the complex flow geometries encountered in such problems. Thus, an attempt has been made to develop a model using ANSYS Fluent 16.2 to investigate the heat transfer rate and solid-liquid interface visualization of PCM filled in different shapes of cavity. It is found that triangular shaped macro encapsulated PCM melts faster than square and circu- lar shaped encapsulated PCM.

Facile Synthesis of Al@Al2O3 Microcapsule for High-Temperature Thermal Energy Storage

Preparation of high-temperature encapsulated phase change materials (PCMs) is very challengeable due to the high chemical corrosion of liquid metal and the high thermal stress caused by volume expansion. We herein report a novel strategy for preparing high-temperature Al@Al2O3 PCM. In this method, nano Ni species are loaded on the surface of Al spheres, which could accelerate the oxidation of the surface Al layer during calcination in air. By monitoring the oxygen consumption and the exothermic changes during the oxidation process, it is found that the oxidation of Al follows a stepwise process which results in the formation of a layered Al2O3 shell. The high oxygen consumption and relatively low activation energy (149–156 kJ/mol) suggests that the surface nickel species significantly accelerate the low-temperature (<650 °C) oxidation of aluminum. The Al@Al2O3 composites show a well-formed core–shell structure with 60–68 wt % of core fraction, high latent heat (289–312 J/g), and mitigated supercooling per...

Modelling and experimental study of latent heat thermal energy storage with encapsulated PCMs for solar thermal applications

The variability in solar radiation creates a gap between energy demand and supply, which necessitates the use of efficient thermal energy storage for bridging the gap to make the solar thermal power plant a viable solution for continuous power generation. In this work, a mathematical model of encapsulated phase change materials (PCMs) based latent heat thermal energy storage (LHTES) is developed considering simplified non-equilibrium two energy equations coupled with enthalpy technique to analyse the transient variation in heat transfer fluid (HTF) temperature at the outlet of LHTES and PCM temperature. Experiments on a spherical capsule reveal melting and solidification behaviour of PCM from the measured temperature field. A lab-scale LHTES is designed and fabricated to evaluate the effects of charging temperature, discharging temperature and flow rate on thermal performance of the LHTES during charging and discharging operations. Detailed parametric study on capsule diameter and porosity shows that the energy stored and extraction are faster for smaller capsule diameter and higher porosity. The maximum efficiency of the storage in this work is found to be 75.69% for charging and discharging inlet HTF temperatures of 180 and 120 °C, respectively and flow rate of 8.2 lpm.

Using renewables coupled with thermal energy storage to reduce natural gas consumption in higher temperature commercial/industrial applications

Abstract In the current study the feasibility of using solar-based renewables coupled with thermal energy storage (TES) to displace gas for heating was explored. To assess the feasibility, a numerical model of an air-based encapsulated phase change (EPCM) storage system was developed, validated, optimised, and economically costed. The optimised air-based EPCM system utilised a high and low melting temperature phase change material (PCM) with a sensible storage filler. It was found that a capsule radius of 10 mm and PCM volume of 13% resulted in the lowest cost of discharged thermal energy of $25.55/kWh when storage effectiveness and pumping power was considered. This system was then coupled to solar data for Adelaide, South Australia, to simulate the performance of a 1 MWt heat load over a year. By solving an hourly system generation and demand profile, it was found that a concentrated solar thermal (CST) and photovoltaic (PV) system coupled with TES was able to economically reduce gas consumption by 45–65% when the price of gas was $30/GJ. By employing near-term cost estimates for CST and PV systems coupled with TES, it was found that gas consumption could be reduced by similar amounts with a gas price of $20/GJ.

Melt-front propagation and velocity profiles in packed beds of phase-change materials measured by magnetic resonance imaging

Fluid flow and heat transfer characteristics in packed beds are of importance in many fields of study. However, experimental characterization and model validation of the numerical methods used to study packed beds is difficult. Utilizing nuclear magnetic resonance (NMR), we present a novel approach to track the melt front (and thus melt temperature) in a non-isothermal packed bed. The approach tracks the melt front through packed beds of wax encapsulated phase change material (PCM) by taking advantage of the change in T2 associated with phase change. Additionally, velocity maps of the flow in the pore space of the same packed bed sample are obtained. The approach uses the 1H signal from the PCM packing and the 19F signal from the flowing fluid to allow monitoring of both particle bed structure/phase and flow velocities in the same sample with the use of a dual tune coil. Together, this information can be used to probe the heat transfer characteristics of packed beds experimentally.

Lattice Boltzmann simulation of flow and heat transfer evolution inside encapsulated phase change materials due to natural convection melting

A comprehensive study of the melting process inside a capsule can potentially take full advantages of latent heat of phase change materials (PCMs). The present study was devoted to the problem of complex interaction of natural convection and melting of PCMs inside a spherical capsule under different sizes. The numerical results, simulated by lattice Boltzmann method (LBM), were compared with experimental data and published simulations. The results showed that LBM presented desirable accuracy compared to traditional computational fluid dynamics (CFD) methods. Then, the effects of non-uniform PCM properties, expressed by the solid/liquid thermal diffusivity ratio, on the melting rate were found to be nonlinear in different melting stages. The non-dimensional fully melting time reduced with the increase of the surface temperature and the capsule size, and the former compared to the latter could have a greater influence on the melting rate. Moreover, the non-dimensional fully melting time reduced when increasing of the capsule diameter at the macro-scale; while there was a near-invariable non-dimensional fully melting time when the capsule size was changed at the micro-scale. The good understanding of the phase change process inside the capsule would provide essential information to develop a multi-scale model of microencapsulated PCM slurries.

Design and analysis of PCM based radiant heat exchanger for thermal management of buildings

The present study focuses on the thermal performance of encapsulated phase change material (PCM) based heat exchanger for thermal management of building in Indian conditions. Heat transfer characteristics of the encapsulated PCM based heat exchanger is investigated experimentally and compared vis-à-vis thermally activated roof and radiant panel systems. Experiments are performed on a scaled down concrete cubical test chamber with a window facing the north direction. It is found that the encapsulated PCM based heat exchanger is able to reduce heat gain of the test chamber by approximately 50% as well as mean air temperature by more than 6 °C. The encapsulated PCM based heat exchanger is found more beneficial in flattening the peak for longer duration and reducing the fluctuation in mean air temperature of the test chamber. The developed building simulation model is validated with the experimental results, which is used in parametric studies to identify the effect of different parameters on the average air temperature in the test chamber. The duration of stabilization period can be kept constant by increasing PCM thickness linearly with the increment in thermal load of the test chamber. The inlet water temperature to the PCM based heat exchanger is found to be the most influential parameter affecting the mean air temperature of the test chamber, whereas thermal conductivity of walls and length of pipe in heat exchanger have minimal effect comparatively. Further, a ground heat exchanger is incorporated with the encapsulated PCM based heat exchanger to investigate the effectiveness of ground heat exchanger in discharging the heat load carried by the working fluid.